1. Field of the Invention
The present invention is directed to pressure gauges and more particularly, temperature compensated pressure limited gauges.
2. Description of the Prior Art
Various forms of temperature compensated guages have been utilized in the prior art, for example, the aircraft industry utilizes gauges to determine whether the pressure of a fire extinguisher has leaked from a container. An example of this form of gauge is disclosed in U.S. Pat. No. 3,735,376 and is directed to determining the relative pressure condition between two containers. Another pressure differential gauge with both force compensating and temperature compensating features is disclosed in U.S. Pat. No. 3,576,412 and is utilized for determining the useful life of a blade spar of a helicopter rotor. Various other forms of temperature compensated pressure indicators for determining leakage from a high pressure container are known in the prior art.
The desirability of having a pressure gauge that is temperature compensated is particularly important in the aircraft industry. The pressure gauge on an aircraft can be subjected to rapid temperature changes and the reliability of a temperature compensated pressure gauge is contingent upon its response to temperature changes. A false alarm in a cockpit of an aircraft can cause a commercial airline to be grounded. An aircraft obviously operates at ambient surface temperatures and within minutes can be flying at subzero temperatures miles above the earth. In addition, vibration and mechanical stresses are frequently applied to the pressure gauge.
An example of a requirement of a temperature compensated gauge can be found in the Boeing 747 aircraft which requires a pressure gauge to monitor the hydraulic fluid in the aircraft's brake system. In normal use, the pressure can be approximately 200 psi; however, when the emergency braking system is activated, to lock the wheel rotation under certain operational modes such as when the aircraft is subject to full engine power, the hydraulic fluid pressure can reach 3000 psi. The pressure gauges that have been utilized in such a system have generally employed ball check valves to limit the application of the pressure to the gauge. The pressure gauge itself is required to be extremely accurate over a limited pressure range, and accordingly, this prohibits designing the gauge to be fully responsive throughout the entire range of pressure to which the gauge may be subjected. Generally, a bourdon coil is designed to sustain a 150% overpressure without damage. As known in the prior art, the more limited the range of the gauge, the easier it is to provide an accurate linear output. Other forms of pressure limited gauges can be found in U.S. Pat. No. 3,789,219.